Field of the Invention
This invention is related to a mine roof support and, more particularly, to a yieldable prop with a yieldable insert.
Description of Related Art
Mine roof supports are used in underground mining operations to support the rock strata that define the underground opening. One type of mine prop, typically referred to as a sand prop, utilizes two steel pipes with one nested inside the other. The inner pipe is filled with “sand”, particulate matter such as ceramic beads having a 0.050 inch diameter, and includes a steel washer at the bottom of the pipe. The two pipes utilize a telescoping arrangement to extend between the roof and floor of the mine opening. When the inner pipe is moved upwardly relative to the outer pipe, the sand flows from the inner pipe and through a hole in the steel washer into the outer pipe, which sets the height of the prop and creates a load bearing structure. This type of sand prop arrangement is typically rated for 30, 60, or 100 tons. Under loading, these conventional sand props typically begin to buckle and fail after approximately 1.5-2 inches of displacement.
Another type of sand prop, which utilizes a similar telescoping arrangement, is yieldable by deforming the outer pipe when placed under a predetermined load. This allows the sand prop to maintain a certain load while undergoing displacement.
Other types of mine roof props are yieldable by using an arrangement having an inner conduit slidably mounted into an outer conduit and held in position by a clamp assembly. As a compression load, e.g., a shifting mine roof, acts on the prop, the first tube slides into the second tube. The force of the clamp assembly typically controls the load that the prop can take before it compresses.